1. Field of the Invention
The present invention generally relates to an apparatus and method useful for actuating a valve. The present invention is particularly useful for actuating a valve incorporated in a drill string for use in a pressure pulse telemetry system. More particularly, the present invention relates to an apparatus and method for adjusting the actuating force applied to a valve in response to a value characteristic of the minimum force necessary to actuate the valve.
2. Description of the Background
The desirability of logging a borehole during or immediately after drilling has long been recognized by those associated with drilling operations. However, borehole logging for many years was exclusively performed by wireline tools lowered into the borehole after removal of the drilling apparatus therefrom. These wireline logging operations, requiring the tripping of the drill string, resulted in lost drilling time and greatly increased costs. Further, changes in various formation characteristics occurred during the delay between the actual drilling of a formation and the performance of these wireline logs. For example, leakage of drilling fluids or formation fluids across the borehole wall during this delay time often resulted in the production of inaccurate and incorrect logs.
For many reasons, including those set forth above, those skilled in the art have long recognized the desirability of performing borehole logging operations while drilling. However, in recent years, there has been significant interest in the development and use of measurement-while-drilling (MWD) systems. Only recently have appropriate tools and methods to perform logging operations while drilling became available. The actual measurement tools must be resistant to the harsh environment created by the constantly vibrating drill string and the prolonged exposure to borehole conditions. Further, these tools must be sufficiently strong to withstand the stresses in the drill string and sufficiently small to not interfere with the operation of the drill string and its associated downhole systems.
MWD systems require apparatus and methods for transmitting the measured data to the surface. Although it is theoretically possible to store the data in a microcomputer or other downhole data storage device for transfer to appropriate data processing devices at the surface upon retrieval from the borehole, these systems have not found widespread use. In order to maximize the benefits from MWD systems, it is necessary to transmit the data immediately to the surface for analysis. Contemporaneous analysis permits the drilling operator and geologist to immediately detect changes in the bottom hole conditions and to make any desirable or necessary adjustments in the drilling operation. Accordingly, apparatus and methods for telemetering MWD data to the surface are necessary. Telemetry systems employed have included systems for transmitting electrical signals through electrical conductors embedded in or on the drill string, systems for transmitting acoustic signals through the drill string or the drilling fluids and systems for imparting measurable pressure pulses to the drilling fluids.
Pressure pulse telemetry systems include apparatus and methods for imparting either negative or positive pressure pulses to the relatively constant drilling fluid pressure in the central bore of the drill string. An exemplary apparatus and method for telemetering information from a borehole to the surface by negative pressure pulse telemetry is disclosed in U.S. Pat. No. 4,078,620 which is incorporated herein by reference. This exemplary system discloses a system for venting drilling fluid through a passage in the wall of a drill sub from the interior of the sub to the annulus in order to impart negative pulses to the pressure of the drilling fluid in the drill string. These negative pulses transmit coded information from the borehole location to the surface where the negative pulses are detected and the data decoded.
In a downhole pressure pulse telemetry system employing an electrically operated valve or pulser, the force required to actuate the valve varies with the downhole differential pressure across the valve. The power supply must be designed to provide an energy output to produce an actuating force sufficient for the worst expected condition. Because energy is wasted in all but the extreme condition, these systems suffer from significant energy waste. This wasting of energy in significant quantities is detrimental for two major reasons. The wasted electrical energy requires the use of a larger power supply than is actually necessary to operate the valve. Power supplies generally comprise downhole batteries or electrical generators. Because it is desirable to maintain the power supply as small as possible, it would be advantageous to minimize this wasted energy. Further, the wasted mechanical energy generated by applying a constant, maximum actuating force to the valve must be dissipated in the pulser assembly. This energy is dissipated as excess heat and as accelerated wear through fatigue and breakage as the result of unnecessarily severe shock and vibration to the moving components of the solenoid and valve assembly, both contributing to a shortened lifetime for the assembly.
Because the downhole pressures typically encountered by these systems are quite high and because the pressure differential between the interior and exterior of the drill string in these locations is also quite high, the design and operation of the telemetry signaling valve is critical. These valves must seal accurately to prevent fluid leakage, they must be fast acting to produce sharp pressure pulses and they must require minimum energy to operate. The successful, long-term operation of these systems has suffered from difficulties in meeting the above requirements.
Whether the electrical power is supplied by a downhole battery or by an electrical generator in the drill string, it is desirable to minimize the electrical needs of the valve system in order to prolong battery life or minimize the size of the needed generator. Others skilled in the art have attempted to solve this problem by various means. For example, U.S. Pat. No. 3,958,217 discloses a positive pulse telemetry system employing a small input signal to operate a pilot valve whereby the main telemetry valve is then operated by pressure differentials created in the mud stream itself. U.S. Pat. No. 4,336,564 discloses a negative pulse telemetry system including solenoid control circuitry for initially providing a large current to the solenoid to open the valve, for reducing the current to a much smaller value to hold the valve open and for interrupting the current to close the valve. U.S. Pat. No. 4,351,037 discloses a system employing back coupled solenoids to open and close the valve so that the only energy required is that necessary to actuate the valve. These systems have attempted to minimize the total energy requirement necessary to operate the valve. Of particular concern was the need to minimize the long term energy necessary to maintain the valve open in order to prevent burn up of the opening solenoid.
Although increased pressure differential across the valve in a negative pressure pulse telemetry system may be useful to prevent leakage through the valve, the force necessary to actuate the valve is also directly related to the pressure differential across the valve. Because it is necessary to ensure that these pressure pulse telemetry systems function under the most severe borehole conditions expected, sufficient force and power to actuate the valve under the most severe, expected conditions must be available.
Accordingly, the valves in pressure pulse telemetry systems have been overpowered to ensure operation under the most severe, expected conditions. By always operating the valve with a force sufficient to overcome the most severe, expected conditions, the lives of the various valve and electrical circuit components have been shortened by dissipation of the excess mechanical and electrical energy applied in most operations. Although the above patents disclose attempts to minimize the total energy requirements necessary to operate pressure pulse telemetry valves, they do not disclose attempts to minimize the initial force applied to actuate the valve to minimize this wear.
Accordingly, there has been a long felt but unfilfilled need within the borehole logging industry for an apparatus and method useful in pressure pulse telemetry systems for actuating the pressure pulse imparting valve with the minimum necessary force in order to both conserve critical downhole energy supplies and minimize the wear on valve and circuitry components.